In recent years, stainless steel jewellery has become more and more important and is sold on an ever increasing scale. Watches made of stainless steel, in contrast, have been known already for a fairly long time and appear correspondingly widely on the market. However, the austenitic steel used at present, both in the sphere of watches and jewellery, has the extremely negative property that this stainless steel, in contrast to silver, gold or titanium jewellery, has a high alloy component of nickel which gives this material very negative aspects with respect to the production of e.g. watches and/or jewellery and/or implants. The greatest disadvantage hereby is the fact that most people react to nickel with allergic reactions and/or sensitivity problems and they must consequently avoid nickel-containing materials of any type on or in the body in order to be able to prevent allergies.
A further disadvantage of the austenitic stainless steels used to date for stainless jewellery and/or watches and/or implants, in addition to the high and very negative nickel content, is that the scratch-resistance thereof, in particular the surface hardness thereof (with approx. 200 HV), can in fact be increased by means of hardening methods which are known in general and described subsequently in order to reduce scratching of the manufactured products, e.g. during use or when worn, but has, at the same time, the extreme By means of known processes, such as e.g. TiC, TiN, PVD, DLC . . . , a hard material layer which is in fact very hard but only very thin (a few μm) can in fact be applied on the surface of the austenitic steel. However, these thin layers have the disadvantage that, during mechanical loading, above all during mechanical point loading, they break down or flake off. The reason for this resides in the fact that the hardness decreases abruptly under the hardened layer to the initial hardness (approx. 200 HV) of the steel used. This effect is generally known in the literature as “the eggshell” effect.
Furthermore, these austenitic steels can in fact be hardened permanently with a hard and better impact-resistant surface (up to approx. 1800 HV) by the generally known low-temperature process “Kolsterising”. Carbon at below 300° C. is hereby diffused in during a process duration of 5 to 6 days. The disadvantage of this method is however the extremely long process duration and the very high production costs associated therewith. In addition, the relatively small layer thickness at up to at most 33 μm is also extremely negative here since the hardness inside this gradient-associated layer decreases relatively rapidly to the initial hardness of the austenitic steel (approx. 200 HV). A further great disadvantage of these hardened or unhardened austenitic steels is their very high nickel content.
One hardening process for a martensitic steel (AISI 410) is known from Corrosion Science 48 (2006) 2036-2049 by C. X. Li et al. relating to a plasma-nitriding process in the low-temperature range between 420-500° C. It is disadvantageous here again that the applied hardness at above 1000 HV decreases almost abruptly, i.e. with a virtually abrupt drop and clearly detectable phase boundary, to the initial hardness (eggshell effect), which has the result that collision or impact energy, e.g. in the form of point loading, can only be partially absorbed, i.e. with poor impact-strength resistance and hence breaking down of the hard layer is very probable. Furthermore, it is known from Davis et al., ASM Handbook, Volume 4, Heating Treating 1991, AMS International US, to nitride also steels of the type AISI 430 and 460 in the low-temperature range up to at most 595° C.
In the case of these cited low-temperature processes (nitriding and plasma-nitriding) for the indicated steels in the temperature range up to at most 595° C., in fact nitrogen diffuses into the steel surface but no phase conversion/austenitising takes place. A further great disadvantage of these steels after treatment thereof is that they are not corrosion-resistant. In addition, the very high process times between 20 and 48 hours and the significantly higher production costs, associated therewith, are disadvantageous.
To date, ferritic chromium steels have been seen as non-hardenable by means of a heat treatment on the basis of the too low carbon content according to the general state of the art, and above all—in comparison with austenitic steels—also as non-corrosion-resistant steels.
Martensitic chromium steels could in fact be hardened previously by specific hardening processes, such as e.g. case hardening with carbon, but the free chromium component and hence the corrosion-resistance thereof is significantly reduced.
For this reason, there is still a great need to improve a biocompatible material made of stainless steel for all products which can be worn on or borne inside the body such that the chromium steel used is distinguished by approximately the same production costs in addition to the required important freedom from nickel and simultaneous excellent scratch-resistance/hardness and corrosion-resistance.
Since with all types of jewellery, watches and implants or all products which are worn on or borne partially inside or completely inside the body, direct and partially also permanent body contact with the material used exists, the requirement to use exclusively a nickel-free material in order to avoid allergic reactions or sensitivity problems is particularly high. In order in addition to prevent or to reduce scratching and rusting due to wear and use, the requirement exists in addition to use a highly scratch-resistant and corrosion-resistant material for all types of jewellery, watches and implants. In order to ensure the profitability of the objects according to the invention, such as jewellery, watches and implants, the production costs should not be increased but advantageously remain approximately the same.
In the prior art described above, hard material layers can in fact be applied on the presently used austenitic steels via TiC, TiN, PVD, DLC . . . , or by Kolsterising, for biocompatible materials (such as e.g. watches/parts for watches, all types of jewellery and implants), however the layer thicknesses produced therewith are very thin and break down very rapidly or flake off upon mechanical point loading because of the abrupt decrease in hardness to the initial hardness of approx. 200 HV. In this case, not only are the applied layers very thin and, without a supporting transition layer, disadvantageous, but also the required additional extremely long process times and the significant increase in production costs associated therewith. For biocompatible materials which are worn on or borne partially inside or completely inside the body, such as e.g. all types of jewellery, watches/parts for watches and implants, economical production with these methods is virtually precluded.
In the case of the described hardening processes (nitriding and plasma-nitriding) for martensitic steels, the loss of corrosion-resistance is in addition very disadvantageous and hence precludes application entirely for the products according to the invention.